Replace Maybe with Either ...smth... in BiMap

[chshersh]

This is needed to give better error messages.

[willbasky]

Subject is about

data BiMap a b = BiMap
    { forward  :: a -> Maybe b
    , backward :: b -> Maybe a
    }

and all derivations?

[chshersh]

@willbasky Yes, that's right. I assume that after refactoring this should look something like:

data BiMap e a b = BiMap
    { forward  :: a -> Either e b
    , backward :: b -> Either e a
    }

And then all current BiMaps will have the following type:

type BiToml = BiMap SomeTomlError